Many printers have discovered that thermography can add another dimension to their business. Thermography today is no longer just for stationery, business cards and announcements, but has emerged as an art form in its own right. Used on its own, or as an adjunct to lithography, foil stamping, embossing and silk screening, it has become an extremely useful tool for graphic designers and artists. Other applications include greeting cards, labels, tags, annual reports, report covers, packaging and posters.
Thermography is an established procedure whereby raised printing that imitates copper plate engraving or stamping from any type of printing process is more easily accomplished using an offset or other conventional printing process. Printed sheets from a conventional printing press drop onto a conveyer where a resin or powder, having the characteristic of melting under the effect of heat, is vibrated onto them. Excess powder may be continuously removed by vacuum suction or by hand and recycled, except where the powder has adhered to the wet ink. The printed and powdered sheet then typically passes through a tunnel-type oven, where it is heated to melt and fuse the powder grains into a raised film. At the exit, cold air is blown onto the sheet to cool and set the viscous raised film so as to prevent sheets from sticking together or smearing. Two examples of such thermographic printing procedures are U.S. Pat. Nos. 5,699,743 and 5,098,739.
It is known in the art that the granule size of the powder used in part determines the thickness of the relief film, or raised print, up to a certain maximum height. Larger powder granules are used to obtain greater heights of raised print. Thermographic raised print on the order of 0.01 inches in height may be obtained according to the methods taught by U.S. Pat. No. 5,699,743 by printing an ink line about 1/16 to 1/8 inch in width, depositing 20 to 50 mesh powder thereon, and heating at the appropriate time and temperature to fuse the powder without completely melting it. Generally, smaller particle sizes of powder are used when lesser heights of the raised print are desired.
There are two distinct types of thermographic powder generally used: transparent and opaque. Transparent powders generally include high gloss, semi-gloss, and semi-dull. Opaque powders typically include metallics, such as gold, silver, and bronze; white; and the relatively new pearlized powder. High gloss is the most commonly used powder for all thermographic applications.
There are five conventional granulations for use on lines ranging in thickness from "fine line" to "heavy solids." Fine lines require the finest granulation typically described as fine as flour, while heavy solids require a coarser granulation typically described as loose as sugar. Semi-gloss, dull and semi-dull powders are primarily selected by designers looking for special effects. They generally provide less shine than the high gloss powders, but retain a similar "feel" and raise. The metallic and white opaque powders, on the other hand, are typically difficult to work with. Thus, thermography shops generally run only high gloss powders.
Thermographic printing has typically been achieved by printing wet ink on a substrate using flat conveying devices to move the printing substrates (e.g., sheets of paper) under a hopper that drops powder thereon. Conventional thermographic printing over the last 100 years or so has typically only been able to achieve printing speeds of 60-70 ft/min. and occasionally as high as 100 ft/min., although it is unclear if this speed can be maintained on a continuous basis. Moreover, the thermographically printable area has been limited to approximately 12 inches in width. Reports have been made of printable widths on the order of up to 20 inches. Offset and other types of conventional printing often have used a cylindrical printing apparatus that may permit higher printing speeds. The apparatuses used in several conventional flat printing and flat color printing on paper and fabrics are described as follows.
U.S. Pat. No. 537,923 discloses an apparatus for producing designs on paper having a stencil sheet cut with the pattern of the desired design, vessel(s) for holding and delivering the inks or colors, and a blast apparatus for delivering the inks or colors through the stencil sheet to come into contact with the paper or other surface upon which the design is desired.
U.S. Pat. No. 2,049,495 discloses a printing apparatus for continuously replenishing a supply of ink to a material which is to be imprinted. The ink supply is provided from within a cellular cylinder to fill the pattern running along the sealing surface of the cylinder.
U.S. Pat. No. 2,334,909 discloses a press roller to print on fabric at relatively high speeds by supplying ink of different colors to the interior side of a stencil, which has openings through which the ink passes to print on the fabric. Also disclosed is an ink or color holder in contact with the stencil that permits the loader to be filled externally from the press and swung into the color holder without the need to stop the press.
U.S. Pat. No. 3,613,635 discloses a spot printing apparatus for printing a powder onto a substrate including a perforated hollow roller, means for rotating the roller, and a hopper means within the roller for holding the powder. The perforated hollow roller has a discontinuous pattern of holes for depositing the dry powder.
U.S. Pat. No. 5,713,275 discloses a stencil printing machine having a holding device for holding the perforated stencil sheet, an ink supply device, a printing sheet conveying device, and an air ejection means for ejecting air to the stencil sheet from within and causing the ink to pass through the image on the stencil sheet and transfer to the printing sheet.
These conventional printing devices are generally directed to printing flat patterns or text on fabrics or paper. It was believed that cylindrical rollers were not suitable for the powdering stage of thermographic printing for various reasons, e.g., loose powder on the substrate would be more likely to smear. As thermographic printing has gained commercial success in various printing endeavors, however, it has become desired to improve the efficiency and quality of thermographic printed products. Thus, it is desired to have an apparatus capable of providing a thermographic printed product that is capable of high speed use and has multiple colors on different parts of the product while still maintaining the high quality achieved by conventional thermographic printing.